Remove OstreeTlsCertInteraction bits from introspection

We filter out everything named `-private.h` from scanning,
which differs from the gtk-doc exclude.  Eventually this will
be solved when we switch to the new gir-based docs.

Came up in https://github.com/ostreedev/ostree-rs/pull/34#discussion_r723337772

apidoc/Makefile.am

@@ -87,7 +87,7 @@ IGNORE_HFILES= \
 	ostree-repo-pull-private.h \
 	ostree-repo-static-delta-private.h \
 	ostree-sysroot-private.h \
-	ostree-tls-cert-interaction.h \
+	ostree-tls-cert-interaction-private.h \
 	$(NULL)
 
 # Images to copy into HTML directory.